Various thickeners exist and are already used for these purposes. Natural products such as guar gum or corn starch are known in particular, the drawbacks of which are those inherent to natural products, such as price fluctuations, supply difficulties and random quality.
Synthetic polymers in powder form, mainly polyacrylic acids, are also widely used but have the drawback of requiring neutralization when they are used, since they only develop their viscosity from a pH &gt;6.5 and they are often difficult to dissolve.
Synthetic thickening polymers in the form of an inverted latex, that is to say one in which the continuous phase is an oil, are also known. These latices dissolve extremely quickly; the polymers contained in these inverted latices are usually acrylamide/alkali metal acrylate copolymers or acrylamide/sodium 2-acrylamido-2-methylpropane-sulphonate co-polymers; they are already neutralized and when they are dissolved in water, for example to a concentration of 1%, it is observed that the pH is generally above 6.
However, acrylamide/sodium acrylate copolymers do not develop any appreciable thickening properties when the pH is lowered below 6; on the other hand, the acrylamide/sodium 2-acrylamido-2-methylpropane-sulphonate copolymers described in EP 0,503,853 retain an appreciable thickening capacity even at pH 4.
However, such copolymers have monoacrylamide contents which, although extremely low, could result in making them impossible to use in cosmetics in the near future, following changes in the European legislation on hazardous substances.
The Applicant has thus been concerned with the synthesis and development of polymers that thicken, even at acidic pH, in the form of an inverted latex, without using monoacrylamide.